This invention relates to yellow azo dye compounds, and more particularly, to dichroic liquid crystal compositions containing the yellow azo dyes.
Yellow dyes having improved properties are desirable for various applications including the dyeing of textiles and the coloring of plastics and liquid crystal displays. Among those properties which are desirable for dyes, especially for dyes used in liquid crystal displays, are photostability of the dye, suitable extinction coefficient of the dye, solubility of the dye in liquid crystal hosts and light absorption in the desired spectral region. In liquid crystal displays, yellow dyes are especially useful for modifying the color of other dyes or as additives for black dichroic formulations. The fabrication of black liquid crystal displays poses a particular problem in that no black dichroic dyes are known, and black liquid crystal displays are made by mixing dichroic dyes of various colors in proper proportions to obtain a material which absorbs light radiation in the spectral range of about 400-700 nm resulting substantially in a black color. The dyes used in such mixtures must have a high order parameter, preferably at least 0.75, to ensure satisfactory brightness and contrast ratio. Heretofore, the only available yellow dyes of high order parameter contained Schiff base linkages within the dye molecule, and the presence of a double or triple bond in the base unit of the dye (exclusive of substituted groups and derivatives), leads to chemical and/or photochemical instability of the material.
Dye compounds suitable for use in solution with a liquid crystal material and containing azo linkages rather than Schiff base linkages in the dye molecule, are well known in the prior art. In U.S. Pat. No. 4,145,114, there is a disclosure of a substantial number of dye compounds suitable for use in liquid crystal materials and having an azo or azoxy linkage of the following general formula: ##STR2## or a substituted derivative of the foregoing formula in which any one of the positions on any one of the benzene rings is substituted by halogen, methyl, halogen substituted methyl, or methoxy; wherein A is an azo or azoxy linkage group; n.sub.1 and n.sub.2 are each integers in the inclusive range 0 to 4; and Y.sub.1 and Y.sub.2 are each one of the following groups: (i) hydrogen; or (ii) OR where R is an alkyl or arylalkyl group. Although the general structure of the dyes of U.S. Pat. No. 4,145,114 avoid the Schiff base likages and thereby eliminate the chemical and/or photochemical instability disadvantages of materials containing the Schiff base linkages, it is always desirable to improve azo dyes and to improve liquid crystal displays containing azo dyes.
The present inventors describe in U.S. Pat. No. 4,122,027, certain dichroic dyes of the 4,4'-bis(substituted naphthylazo)azobenzene type, each having three azo bonding groups, namely seven dyes having the general formula: ##STR3## and in U.S. Pat. No. 4,179,395, certain dichroic dyes are described having the general formula: ##STR4## In pending application Ser. No. 175,003, filed Aug. 4, 1980, the present inventors describe certain yellow azo dichroic dyes having the general formula: ##STR5## in which R and R' are acyl. Copending application Ser. No. 175,002, also filed Aug. 4, 1980 teaches certain yellow azo dichroic dyes of the general formula: ##STR6## in which R and R' are acyl. Also, in application Ser. No. 175,004, also filed Aug. 4, 1980, yellow azo dyes having the general formula: ##STR7## where R and R' are acyl are described. The above applications and patents are incorporated herein by reference in their entireties.